The present exemplary embodiment is generally related to MEMS devices and methods of manufacturing such devices. More particularly, the exemplary embodiment is related to a MEMS device comprising at least one shape memory material, such as a shape memory alloy layer and at least one stressed material layer. The device can be utilized as, for example, an actuator, a microswitch, a micropump, a microvalve, and a non-destructive fuse-type connection probe.
Micro Electronic Mechanical Systems (MEMS) are microscopic devices and machines fabricated by integrating mechanical or hydraulic functions with electrical functions such as provided by computing/logic circuitry. MEMS technology has been widely utilized in a variety of industries. For example, a MEMS device can function as a sensing device, which combines a sensor and logic circuitry to perform monitoring functions such as pressure and acceleration measurement for deployment control of airbags in cars. A MEMS device can also function as a micromirror or waveguide to guide wavelengths in optical switches and tunable lasers. Typically, MEMS devices are processed with the same materials and methods used to make integrated circuits (IC's). Generally in producing a MEMS device, a tiny mechanical component such as a sensor, a valve, a gear, a mirror, and/or an actuator is incorporated in an electronic microchip such as a semiconductor chip.
An actuator can be in the form of a MEMS device that, in responding to a signal received from a control system, can change and/or maintain the position of an element such as an end-effecter in performing a task. Shape memory alloys (SMA) have great potential as actuators due to their very high displacement and energy density. However, for most SMAs, the shape memory effect is a “one-time-only” actuation (one-way shape memory effect). That is, once a SMA is recovered to its original, un-deformed state by heating to above the martensite-austenite phase transition temperature, it will retain the un-deformed state and can not be automatically returned to its previous deformed state upon cooling. As illustrated in FIG. 1, first, under a low temperature (T1) environment, the SMA material exists in its martensite phase and is therefore un-deformed. At this low temperature, if a mechanical stress is applied on the SMA, it will be deformed. As the martensite phase has a low Young's modulus and a low yield strength, usually a small stress can induce a large deformation and cause plastic deformation in the material. Next, the material can be heated to a higher temperature (T2) and shifted to the high temperature austenite phase. As the austenite phase has a much higher Young's modulus and yield strength, at the higher temperature (T2) the material will return to its un-deformed state. For one-way shape memory effect, after cooling and returning to its martensite phase, the material will remain in the un-deformed state, a so-called one-time-only actuation.
Two-way shape memory effect is much more desirable in MEMS application since the actuator can be used repeatedly. The effect is illustrated in FIG. 2. The material will “remember” the low temperature (T1) deformed state, and upon sufficient cooling of the material, it will return to its original low temperature deformed state. In order to obtain the two-way shape memory effect, a material needs special treatment and has considerably smaller recovery strain than the material with only one-way shape memory effect, thus most MEMS devices still use one-way shape memory materials.
Most shape memory devices with one-way shape memory materials need to have a mechanical bias acting on the SMA to retain the device in the deformed state at the low temperature martensite phase. FIGS. 3A and 3B depict a known microvalve 10 having such a configuration. The microvalve includes an actuator 20 consisting of a TiNi thin film 22 and a silicon spring 24 bonded to the film 22 as the mechanical bias. In low temperature or cold state (FIG. 3A) the alloy film 22 is deformed by the silicon spring 24 and the valve 10 is in a closed position. When the alloy film is heated (FIG. 3B) such as by applying a voltage across it, the film 22 will recover to the un-deformed state and the valve 10 will be in an open position. After removing the voltage the alloy film will cool and will return to its deformed state, and the valve will be in the closed position again (FIG. 3A).
FIG. 4 illustrates a known micropump 50 having a similar configuration. In FIG. 4, the micropump 50 includes a glass pressurization or evacuation cap 52 that is bonded on top of a shape memory alloy thin film diaphragm 54 so that the SMA thin film 54 is kept in a deformed state at low temperature. The micropump 50 also includes one or more check valves 56. At its low temperature state (either (I) or (II) in FIG. 4) the diaphragm 54 is in a deformed or deflected state. Upon heating, the diaphragm 54 is urged to its un-deformed state (either (III) or (IV)), thereby drawing liquid or fluid into the micropump 50 (designated as arrow A in state (III)) or inducing flow out of the micropump 50 (designated as arrow B in state (IV)). Upon cooling, the diaphragm 54 returns to its deformed state (either (I) or (II)) thereby drawing a liquid or fluid into the micropump (shown as arrow A in state (II)) or inducing flow out of the micropump (designated as arrow B in state (I)).
With regard to FIGS. 3A, 3B, and 4, it is instructive to note that while in a heated state, the SMA films are likely not absolutely flat, and may have some, but much less deformation, as a result of the forces still acting on the SMA films. As can be appreciated from FIGS. 3A, 3B, and 4, complicated structures must be provided to provide the mechanical stress biases, and their fabrication therefore requires numerous processing and assembly steps. Additionally, the size of the resulting MEMS devices is also unsatisfactorily large.
Many other mechanisms have also been explored to add a bias force to the SMA films to induce deformation, especially for SMA/substrate bimorph structures. For example, a microwrapper device was produced by using a NiTi SMA thin film/polyimide bi-layer structure as described by Gill et al. in “Manufacturing Issues of Thin Film NiTi Microwrapper,” Sensors and Actuators A 93 (2001), 148-156, herein incorporated by reference. Due to the large difference between the thermal expansion characteristic of NiTi and polyimide, large residual stress is developed when the device is cooled from the polyimide deposition temperature to room temperature. At room temperature, the NiTi is in martensite phase and has a low Young's Modulus and low yield strength. As a result, the device curls upward. When heating the device to induce the martensite-austenite phase transition, the device becomes flat, since when in the austenite phase the NiTi film has a much higher Young's modulus and yield strength. Similarly, many NiTi/Si bimorph devices are known which utilize the thermal mismatch between the NiTi film and a silicon substrate to generate residual stress, or different residual stresses when the NiTi is in martensite phase and austenite phase, which in turn result in displacement of the device. However, other strategies achieve two-way shape memory effects by using a compositionally graded NiTi bimorphic structure, without an external bias, such as in U.S. Pat. No. 6,689,486 to Ho et al., herein incorporated by reference.
Metal films with internal stresses have been used to make contact probes for packaging and out-of-plane microcoils for RF electronics. The ability of the metal film to be bent in a loop indicates that very large elastic energy can be stored in the film, which in turn suggests its use in high energy or large displacement actuators. Nonetheless, the application of such films in a MEMS actuator is so far limited and unsatisfactory, at least partially, because release of the metal film is also a “one-time-only” actuation. That is, as previously explained with regard to SMA's, after being released and bent, it is very difficult to recover the metal film to its original flat state. While it has been suggested to use electrostatic force to pull the film back to the original state, there are still many problems associated with that approach such as the very high applied voltage that would be needed.
Accordingly, the present exemplary embodiment overcomes the noted problems, and provides an improved MEMS device by combining at least one shape memory alloy (SMA) layer and at least one stressed material layer, and incorporating the layered assembly in a MEMS device.